The present invention relates to a pneumatic radial tire which makes it possible to reduce the weight of a tire without worsening road noise and steering stability.
To preserve the natural environment of the earth, a reduction in the fuel consumption of an automobile is strongly demanded, and a reduction in the weight of a pneumatic radial tire is requested as one method for reducing the fuel consumption. One means useful in reducing the weight of the pneumatic radial tire is to reduce the amount of rubber of a tread in which the amount of rubber used is the largest in the entire tire structure.
However, in the case of a conventional tire, if the amount of the tread rubber is merely reduced, the weight of a crown portion is reduced to increase the natural frequency of the conventional tire which is approximately 90 Hz. As a result, road noise of relatively low frequency band of 80-125 Hz increases, which is transmitted from a road surface to the inside of a vehicle via the tire. Furthermore, since a vibration damping effect lowers as a result of the reduction in the amount of the tread rubber, there has been the problem that road noise becomes still worse.
To prevent the road noise from being worsened by the reduction in the amount of the tread rubber, it is necessary to minimize the increase in the natural frequency of the tire of approximately 90 Hz. One method for reducing the natural frequency of the tire is to lower the vertical rigidity of a side wall portion (casing rigidity) by altering the portions of the tire structure other than the tread. However, if the casing rigidity is lowered, the steering stability becomes worse. In other words, an improvement in steering stability and a reduction in road noise are incompatible, antinomic relationship.